Negotiate the Purple Haze

Pam Smith
By  Pam Smith , Crops Technology Editor
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Purple deadnettle casts a colorful hue over a southern Illinois field north of Metropolis. The beauty has a negative side for farmers. (DTN photo by Pamela Smith)

DECATUR, Ill. (DTN) -- I may have had one foot in the '60s, but my appreciation for Jimi Hendrix rests more with his choice of a song title than the song itself. Each year when farm fields fill with the color purple, I find myself in a haze that hovers between happiness and misery.

Purple deadnettle, followed with a slightly more bluish henbit, are typically among the first weeds of spring. The mild winter and early spring have made for an extraordinary display of both weeds this year, noted University of Illinois weed scientist Aaron Hager.

"It's easy to tell who put down herbicide applications last fall to control these winter annuals," Hager said. "We're seeing some burndown treatments go out to control them this spring in central Illinois, but some are waiting to try to get as much out of their residuals as possible."

It may be striking of beauty, but purple deadnettle, in particular, harbors a nasty side effect beyond competing for resources. A study by Purdue University scientists Valerie Mock, J. Earl Creech and Bill Johnson shows the weed to be the most compatible alternative host for soybean cyst nematode (SCN).

SCN is found in most soybean production regions of the U.S. and is a significant economic threat. Purdue scientists have documented six winter annuals that roll out the welcome mat for SCN. Henbit is also considered a strong host. Field pennycress is a moderate host. Shepherd's purse, small-flowered bittercress and common chickweed are considered weak hosts.

In fact, the Purdue researchers found that in the greenhouse, SCN reproduction on purple deadnettle and henbit was so efficient that it often equaled or exceeded SCN-susceptible soybean.

Purdue research also showed SCN juveniles were present inside purple deadnettle and henbit roots in both fall and spring. However, juveniles were more abundant in spring weed infestations. That finding highlights a management strategy increasingly popular with growers -- controlling these weeds prior to planting either through fall or spring burndown treatments.

In a news release, Iowa State University plant pathologist Greg Tylka said SCN juveniles can't develop in roots at temperatures below 50 degrees F. "But if purple deadnettle, henbit and field pennycress are growing in SCN-infested fields and soil temperatures are greater than 50 degrees F, SCN reproduction and increases in population densities can occur," he said. Tylka added that it takes 24 days to complete a lifecycle at ideal temperatures (76 degrees), while it takes four or more weeks at colder temperatures. So the year and the region factor into whether soil temperatures warm up enough for SCN reproduction to occur on winter annuals in spring months.

This year SCN juveniles have had the equivalent of a prolonged spring break in many regions of the country. Winter annuals are also popular spots for black cutworm moths to deposit eggs.

In interviews, Hendrix gave different answers about the development of the Purple Haze song lyrics. Biographer Harry Shapiro has said it was most likely "a potpourri of ideas" which Hendrix developed over time.

I'll admit to a mixed confusion of thought every time I write about this group of weeds. I understand the agronomic realities, but enjoy the vibrant vibrato these purple flowering weeds send across the landscape. And I'm not the only one -- honeybees and other pollinators are naturally attracted to these early fields as a food source.

When I quizzed Purdue University entomologist Christian Krupke about these weeds, he allowed that they are important plants for many bees, especially solitary ground nesters.

Krupke did ease my bee attitude slightly. He said the lethal doses for most herbicides are extremely high -- noting that the solvents and carriers in the spray formulations are likely to be more harmful to bees than the actual herbicide.

He said timing sprays to avoid bee foraging would be difficult since it would require spray applications at dawn or dusk, or rainy days -- those periods when bees are less active. "Unfortunately, none of those [times] work well for farmers applying pesticides," Krupke said. "When bees are working, they're [farmers] are working."

So while the infusion of purple into spring may put a spell on me (and the bees), Hager said fields that missed controls this spring are good candidates for herbicide applications this fall. "The yellows (cressleaf groundsel and yellow rocket) are starting to come on now," he said. "That means henbit and deadnettle have probably already set seed."

Excuse me while I kiss the sky, as Hendrix sang. The purple haze is clearer when one realizes that in this case, as in many aspects of nature and agriculture, control comes with compromise.

For more information on winter annuals and SCN from Greg Tylka:…

For more information on Purdue's study:…

Pamela Smith can be reached at

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Pam Smith